Semiconductor power switches connected in series with a load and having a semiconductor temperature switch in a gate circuit for controlling the conduction state in response to temperature are known. In order to protect the load from overheating, the temperature switch is commonly heated by a resistor connected across the load or is heated by the load itself by being placed in heat transfer relation therewith. The temperature switch responds above a certain sensed load temperature to shunt gate current whereby the power switch is turned off and the load protected against overheating.
While these devices have been useful for their intended purposes, they are not thermally self-protected with the resultant disadvantage of loss of gate control if the power switch junctions suffer thermal breakdown before load heat activates the temperature switch. In such a case, the power switch junctions have no current-blocking ability and the power switch will remain "on" regardless of whether the temperature switch subsequently shunts gate current; the load is thus no longer protected from overheating and must be manually deenergized to prevent damage thereto.